Washington -LRB- CNN -RRB- The Obama administration 's most significant legislative achievement is now , once again , teetering before the Supreme Court .

The justices are n't weighing the fate of the entire statute this time . In fact , the dispute concerns what Congress meant in just four words in one section of the law . But the impact could be nearly as dramatic .

In 2012 , the Affordable Care Act barely survived a constitutional challenge when Chief Justice John Roberts stunned conservatives and voted along with the court 's four liberal justices to uphold the law .

Health chief : No plan if Obamacare overruled

On March 4 , the justices will gather again to hear a challenge to the law , based on the validity of subsidies in the form of tax breaks , that could affect millions of Americans .

The health care law provides for the establishment of `` exchanges '' through which individuals can purchase competitively priced health insurance . It also authorizes federal tax credits to low - and middle-income Americans to help offset the cost of the policies .

Currently 16 states plus the District of Columbia have set up their own exchanges ; the remaining 34 states rely on exchanges run by the federal government .

Those bringing the case say that the words `` established by the State '' in a subsection of the law make clear that subsidies are only available to those living in the 16 states that set up their own exchanges .

They say the IRS -- one of the agencies charged with implementing the law -- was wrong to offer tax credits to individuals who live in states that only have federally run exchanges . In their eyes , the only exchanges that comport with the letter of the law are those established by the states themselves .

If the court says the IRS rule is invalid , absent some kind of action by the states or Congress , more than 5 million individuals will no longer be eligible for the subsidies , shaking up the individual market .

The stakes are high . Though a ruling against the government would n't nullify the law , it could destabilize it . If millions of Americans were to lose the tax subsidies , and as a result not buy insurance , it would cause premiums to skyrocket in the individual market because there would be less healthy people in the pool .

It would then be up to Congress to fix the law or states to set up their own exchanges . Just Tuesday , Health and Human Services Secretary Sylvia Mathews Burwell warned that if the government loses , it has no plan B to `` undo the massive damage . ''

The challengers say that Congress limited the subsidies in order to encourage the states to set up their own exchanges . But when only a few states acted , the IRS tried to `` fix '' the law and wrote a rule allowing subsidies for those living in states with state-run exchanges as well as states with federally run exchanges .

`` If the rule of law means anything , '' wrote Michael Carvin , a lawyer for the challengers , in court briefs , `` it is that text is not infinitely malleable and that agencies must follow the law as written -- not revise it to ` better ' achieve what they assume to have been Congress 's purposes . ''

Carvin stressed that even though the case is `` socially consequential '' and `` politically sensitive , '' the court must look at what precisely the law says . `` Under those principles , it is clear that the IRS rule must fall , '' he said .

Congressional opponents of the ACA such as Republican Sens. Ted Cruz and John Cornyn agree . In a friend-of-the-court brief , they argued that the executive branch improperly encroached on Congress ' lawmaking function and that the IRS interpretation `` opens the door to hundreds of billions of dollars of additional government spending . ''

Solicitor General Donald B. Verrilli Jr. said that the language at issue is a `` term of art '' and that Congress always intended the subsidies to be available to everyone . Verrilli said it was `` abundantly clear '' that some states would not establish their own exchanges and that if the challengers win , those living in states with federally facilitated exchanges would `` face the very death spirals the Act was structured to avoid . ''

The legislators who actually crafted the law agree with Verrilli . The challengers ' assertion is `` inconsistent with the text and history of the statute , '' they argued .

In addition , 22 states filed a brief saying that if the challengers prevail , insurance markets throughout the country will be disrupted and the law may not be able to operate as a `` comprehensive nationwide program . ''

If the challengers win , individuals living in the 34 states that opted not to establish exchanges would face the loss of their subsidies . In 2014 , more than 5.3 million Americans selected an insurance plan through one of the federally facilitated plans . Enrollment for 2015 is still in progress , but in court briefs the government said that nearly 7 million customers have selected or been re-enrolled in a plan through a federally facilitated exchange and `` coverage throughout the exchanges is expected to continue to increase substantially in the coming years . ''

Those living in states with their own exchanges would not be in danger of losing their subsidies as a result of a ruling against the government . But if the system is destabilized elsewhere , all bets are off .

The plaintiffs in the case are residents of Virginia , one of the 34 states that declined to establish its own exchange . David King , Douglas Hurst , Brenda Levy and Rose Luck all signed declarations with the Supreme Court that they did n't want to purchase ACA-compliant health insurance in 2014 .

The crux of their argument is that if it were not for the tax credits for premiums , they could not afford health insurance and thus would be exempt from the individual mandate to purchase health insurance .

But in order to bring the challenge , the plaintiffs have to show that they have the `` standing '' or legal right to be in court . This means that they have to have been harmed by the government action .

Although the lower courts dismissed the government 's argument that the plaintiffs did n't have standing , the media and others have uncovered information that could raise new questions . It turns out , for example , that two of the plaintiffs are veterans and could be eligible for health care through the Department of Veterans Affairs , though they signed a declaration swearing that they were n't eligible for government insurance . If they were , they would n't have to buy insurance through the exchange .

As things stand now , however , the government has not pressed the issue before the Supreme Court . But the justices themselves might be interested in the issue , which could potentially derail the case .

@highlight

Affordable Care Act finds its future hinging once again on a Supreme Court decision

@highlight

Justices will hear arguments in the case on March 4